The present invention relates to a gear pump comprising rotors disposed in roller bearings and its uses.
For compressors, fans, and pumps, particularly screw-type pumps, it is known from U.S. Pat. No. 2,679,412 to support the shaft by means of roller bearings. Between an inner race and a shaft collar on which a pump rotor is supported, an oil throw-off ring arranged directly next to the bearing, a spacer bush, and a sealing counterring, which rests against the shaft collar, are arranged and are clamped together with the inner race to form a subassembly which rotates together with the shaft.
In a bearing housing and sealing housing which encloses this subassembly, two sliding rings are arranged with the same axes between the roller bearing and the sealing counterring and, stressed by springs supported on the housing, rest against the face of the counter ring facing away from the pump rotor. The two sliding rings bound a ring-shaped space, a lubricating fluid pipe leading into this ring-shaped space.
Furthermore, the use of roller bearings for gear pumps is known per se, in which case reference is made, for example, to the following U.S. Patent Documents: ##STR1##
Pumps of this type may be used for the delivery of media at room temperature as well as, as known, for example, from the EP-A 535 295 which was published later, for the delivery of caoutchouc, which takes place at temperatures of from 90.degree. C. to 120.degree. C.
On the whole, the use of externally lubricated roller bearings for the pump rotors has the important advantage that the bearing of the shaft takes place completely independently of the lubricating characteristics of the delivered polymer. Naturally, this requires an absolute separation of roller bearings and delivered media. In addition, roller bearings arranged outside the delivered flow are much more easily accessible for maintenance and servicing purposes than slide bearings situated in the flow of the molten mass, where a demounting is possible only in the hot condition.
It is an object of the present invention to further develop pumps of the initially mentioned type for the delivery of media at high temperatures and to be able to utilize the above-mentioned advantages also in the case of such a use.
For gear pumps, specifically gear pumps which deliver very viscous fluids, such as polymer meltings with differential pressures in the order of up to approximately 500 bar at temperatures &gt;200.degree. C., it has so far been customary to dispose the rotor shafts in slide bearings lubricated by the media.
The above-mentioned object is achieved by the further development of the initially mentioned pump by providing that, for the delivery of very viscous polymer meltings, bearings surfaces of at least one of inner races, outer races of the bearings and bearing surfaces of roller elements are made of a steel with a high-temperature stability.
In contrast to screw-type pumps, in the case of gear pumps, the largest shaft diameter is, as a rule, smaller than the diameter of the root of the tooth of the medium-delivering gearwheels. From this point of view, the inherent stiffness of the shafts is limited. However, any bending of the shafts has the result that the contact conditions of the mating medium-delivering gearwheels as well as the effect of provided shaft sealing devices will be impaired, whereby the service life of the gearwheels and of the shaft bearings will be reduced.
It is therefore particularly astonishing that the arrangement and construction of the bearing according to the invention makes it easily possible to absorb, by means of rotor shafts supported in roller bearings, the above-mentioned stress caused by pressure and particularly also by temperature.
This relates particularly to the delivery of very viscous abrasive media, as, for example, represented by highly filled polymer meltings. In the case of such delivery media and differential pressure of up to 500 bar, a slide bearing lubricated by a delivered medium is subjected to wear which is no longer acceptable economically. A bearing is provided according to the invention which is completely independent of the lubricating characteristics of the delivered polymer. The reason is that the provided roller bearings no longer come in contact with the delivered medium but are lubricated.
In contrast to the customary bearing technology for gear pump rotors, the use of roller bearings is therefore suggested for the above-mentioned rotor shafts. These roller bearings do not come in contact with the delivered medium itself. This permits in an economical manner the use of the gear pump for very viscous abrasive media, such as highly filled polymer meltings.
While it is taken-into account that the temperatures of the melting may be above 300.degree. C., the bearing according to the invention results in economical service lives.
Preferably, the bearing material in the form of S6-5-2 high speed steel or ceramic material such as silicon nitride (Si.sub.3 N.sub.4) will be used.
In particular, as mentioned above, no external lubricating medium circulating system is provided, but the roller bearings are not lubricated or are lubricated only by lubricant remaining in the bearing. Preferably, a solid-substance lubrication is provided.
Furthermore, in order to minimize the bending of the shafts, the distance between the gearwheel faces and the supporting area of the shaft, determined by the outer contact point in the roller bearing, can be dimensioned to be shorter than determined by twice the median diameter of the shafts along this course.
In particular, when using the gear pump according to the invention for the delivery of a very abrasive delivered medium, such as highly filled plastic masses, one labyrinth seal respectively can be provided between the roller bearing and the gearwheels. As a result, it is prevented, specifically by means of sealing devices, which do not increase the number of components to be provided on the bearing, that delivered medium penetrates from the delivery space into the roller bearings.
Furthermore, as a result of the fact that the labyrinth seals can be tempered by a heat transport medium, the possibility is created to optimize its sealing effect, while taking into account the alternating thermal stress, as caused, for example, by different operating points of the gear pump.
With respect to the construction and the effect of the above-mentioned labyrinth seals, reference is made to "Hydrodynamic Seal for Rotating Shafts", Prof. ret. E. F. Bohn, et al., Chemie-Ingenieur-Technik, 31st Year, 1959, Page 202, an on., as well as to U.S. Pat. No. 4,336,213.
Furthermore, the effect of the labyrinth seal in which the delivery device points from the gearwheels toward the roller bearings is preferred.
A further measure for preventing that delivered medium enters into the roller bearings is implemented by providing, between sealing devices and the roller bearings, an annular gap about the shaft so that possible delivered medium particles penetrating through the seal will fall to the outside through an opening of the gap provided for this purpose.
In the case of such a pump, both pump shafts can be driven separately, for example, by way of an external distributor gear, or only one of the rotors will be driven.
This type of a gear pump according to the invention is particularly suitable for very viscous abrasive media, as represented, for example, by highly filled polymer meltings. Customary fillers and reinforcing materials in this case are, among others, glass fibers, carbon fibers as well as minerals, such as silica sand, ground slate or chalk.